An adiabatic, air-water, co-current, vertically downward bubbly flow was studied to gain a better understanding of interfacial structures and flow characteristics. The experimental test sections were round pipes with internal diameters of 25.4 and 50.8 mm. Flow regime map was obtained using characteristic signals obtained from an impedance void meter, and a neural network-based identification methodology to minimize the subjective judgment in determining the flow regimes. A four-sensor conductivity probe was used to measure the local two-phase flow parameters that characterize the interfacial structures. The parameters measured were: void fraction, interfacial area concentration, bubble velocity, and bubble Sauter mean diameter. Furthermore, a laser Doppler anemometer (LDA) system was used to measure local axial liquid velocity and turbulence. The local profiles of these parameters as well as their axial development revealed the nature of the interfacial structures and the bubble interaction mechanisms occurring in the flow. Based on previous study of interfacial area transport for upward flows, the interfacial area transport equation applicable to downward flow was developed with certain modifications in bubble interaction terms. (C) 2004 Elsevier Ltd. All rights reserved.